1. Field of the Invention
The invention relates to a calender and process, in particular for treatment of a paper web, the calendar having a roll stack of at least three rolls, with at least one double-soft nip formed between two rolls having elastic surfaces.
2. Description of Background Information
A calender is known, for example, from European Patent Application EP 0 748 895 A2. Calendars of this type play mainly two roles in paper manufacture. The first role of the calender is to agglomerate the paper web. The second role is to produce specific surface characteristics, for example, the highest gloss possible and the highest smoothness possible.
The calender known from EP 0 748 895 A2 has, in a roll stack of 6 to 12 rolls, alternatingly a "hard" and a "soft" roll. The hard rolls are heated and have a very smooth surface. They are, as a rule, designed as steel or cast-iron rolls. The smooth surface of a hard roll "imprints" itself on the surface of the paper web, and gives the paper web the desired smoothness and, together with the warmth produced by heating, the desired gloss. The "soft" rollers have an elastic surface and serve primarily to agglomerate the paper web. Since the surface of the soft rolls is elastic, the soft rolls avoid crushing of the fibers of the paper web to a certain extent.
As a rule, it is desirable to smooth both sides of the paper web. Accordingly, in the known calender, both sides of the paper web must be passed over a hard roll with a smooth surface. For this, a "change nip", which is formed by two soft rolls, is required. This change nip has the sole role of altering the sequence of hard and soft rolls (e.g., switching the sides of the web to which the hard roll and soft rolls are applied). The actual effect on the paper web in the change nip is generally considered slight or negligible.
A similar situation also results with other material webs which must be processed in a similar manner, such as a paper or cardboard web.